Lightly donor doped electrodes for high-dielectric-constant materials

ABSTRACT

A preferred embodiment of this invention comprises a conductive lightly donor doped perovskite layer (e.g. lightly La doped BST  34 ), and a high-dielectric-constant material layer (e.g. undoped BST  36 ) overlaying the conductive lightly donor doped perovskite layer. The conductive lightly donor doped perovskite layer provides a substantially chemically and structurally stable electrical connection to the high-dielectric-constant material layer. A lightly donor doped perovskite generally has much less resistance than undoped, acceptor doped, or heavily donor doped HDC materials. The amount of donor doping to make the material conductive (or resistive) is normally dependent on the process conditions (e.g. temperature, atmosphere, grain size, film thickness and composition). This resistivity may be further decreased if the perovskite is exposed to reducing conditions. The lightly donor doped perovskite can be deposited and etched by effectively the same techniques that are developed for the high-dielectric-constant material. The same equipment may used to deposit and etch both the perovskite electrode and the dielectric. These structures may also be used for multilayer capacitors and other thin-film ferroelectric devices such as pyroelectric materials, non-volatile memories, thin-film piezoelectric and thin-film electro-optic oxides.

This application is a Continuation of application Ser. No. 08/040,946,filed Mar. 31, 1993 now abandoned.

FIELD OF THE INVENTION

This invention generally relates to improving electrical connections tomaterials with high-dielectric-constants, such as in the construction ofcapacitors.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with current methods of forming electrical connections tohigh-dielectric-constant materials, as an example.

The increasing density of integrated circuits (e.g. DRAMs) is increasingthe need for materials with high-dielectric-constants to be used inelectrical devices such as capacitors. The current method generallyutilized to achieve higher capacitance per unit area is to increase thesurface area/unit area by increasing the topography, such as in trenchand stack capacitors using SiO₂ or SiO₂/Si₃N₄ as the dielectric. Thisapproach becomes very difficult in terms of manufacturability fordevices such as the 256 Mbit and 1 Gbit DRAMs.

An alternative approach is to use a high permittivity dielectricmaterial. Many perovskite, ferroelectric, or high-dielectric-constant(hereafter abbreviated HDC) materials such as (Ba,Sr)TiO₃ (BST) usuallyhave much larger capacitance densities than standard SiO₂—Si₃N₄—SiO₂capacitors. Various metals and metallic compounds, and typically noblemetals such as Pt and conductive oxides such as RuO₂, have been proposedas the electrodes for these HDC materials. To be useful in electronicdevices, however, reliable electrical connections should generally beconstructed which do not diminish the beneficial properties of thesehigh-dielectric-constant materials.

SUMMARY OF THE INVENTION

As used herein the term high-dielectric-constant means a dielectricconstant greater than about 150. The deposition of an HDC materialusually occurs at high temperature (generally greater than about 500°C.) in an oxygen containing atmosphere. The lower electrode structureshould be stable during this deposition, and both the lower and upperelectrode structures should be stable after this deposition. There areseveral problems with the materials thus far chosen for the lowerelectrode in thin-film (generally less than 5 um) applications; many ofthese problems are related to semiconductor process integration. Forexample, Ru is generally not a standard integrated circuit manufacturingmaterial, and it is also relatively toxic. Pt has several problems as alower electrode which hinder it being used alone. Pt generally allowsoxygen to diffuse through it and hence typically allows neighboringmaterials to oxidize. Pt also does not normally stick very well totraditional dielectrics such as SiO₂ or Si₃N₄, and Pt can rapidly form asilicide at low temperatures. A Ta layer has been used as a sticking orbuffer layer under the Pt electrode, however during BST deposition,oxygen can diffuse through the Pt and oxidize the Ta and make the Taless conductive. This may possibly be acceptable for structures in whichcontact is made directly to the Pt layer instead of to the Ta layer, butthere are other associated problems as described hereinbelow.

Other structures which have been proposed include alloys of Pt, Pd, Rhas the electrode and oxides made of Re, Os, Rh and Ir as the stickinglayer on single crystal Si or poly-Si. A problem with these electrodesis that these oxides are generally not stable next to Si and that thesemetals typically rapidly form silicides at low temperatures (generallyless than about 450° C.).

One difficulty with the previous solutions is that they generallyutilize materials (e.g. Ru) which are unusual in a semiconductor fab.Another difficulty is that a relatively good dry etch for Pt or Ruo₂does not yet exist. As another example, there currently does not exist acommercial chemical vapor deposition process for Pt or Ru. In addition,Pt is normally a fast diffuser in Si and therefore can cause otherproblems. Also, most of the proposed electrode structures requireseveral additional process steps which can be uneconomical. For example,there currently does not exist a commercial chemical vapor depositionprocess for Pt or Ru, nor a commercial dry etch for RuO₂.

Generally, the instant invention uses a lightly donor doped perovskiteas the electrode in a thin-film microelectronic structure. An electrodebuffer layer may also be used as a sticking layer and/or diffusionbarrier and/or electrical connection, if needed. A lightly donor dopedperovskite generally has much less resistance than undoped, acceptordoped, or heavily donor doped HDC materials. The bulk resistivity of atypical lightly doped perovskite such as BaTiO₃ is generally betweenabout 10 to 100 ohm-cm. Also, this resistivity may be further decreasedif the perovskite is exposed to reducing conditions. Conversely, theperovskite can achieve a high resistivity (about 10¹⁰-10 ¹⁴ ohm-cm) forlarge donor concentrations. The amount of donor doping to make thematerial conductive (or resistive) is normally dependent on the processconditions (e.g. temperature, atmosphere, grain size, film thickness andcomposition). There exists a large number of perovskite,perovskite-like, ferroelectric or HDC oxides that can become conductivewith light donor doping.

The deposition of the lightly donor doped perovskite lower electrode maybe performed in a slightly reducing atmosphere in order to minimize theoxidation of the layer(s) underneath it. The subsequent deposition ofthe HDC dielectric material can require very oxidizing conditions, andthe lightly donor doped perovskite lower electrode slows the oxidationrate of the layer(s) underneath it, thus inhibiting the formation of asubstantially oxidized continuous resistive contact layer. Anotherbenefit of this electrode system is that the lightly donor dopedperovskite lower electrode does little or no reduction of the HDCdielectric material.

The disclosed structures generally provide electrical connection to HDCmaterials while eliminating many of the disadvantages of the currentstructures. One embodiment of this invention comprises a conductivelightly donor doped perovskite layer, and a high-dielectric-constantmaterial layer overlaying the conductive lightly donor doped perovskitelayer. The conductive lightly donor doped perovskite layer provides asubstantially chemically and structurally stable electrical connectionto the high-dielectric-constant material layer. A method of forming anembodiment of this invention comprises the steps of forming a conductivelightly donor doped perovskite layer, and forming ahigh-dielectric-constant material layer on the conductive lightly donordoped perovskite layer.

These are apparently the first thin-film structures wherein anelectrical connection to high-dielectric-constant materials comprises aconductive lightly donor doped perovskite. Lightly donor dopedperovskite can generally be deposited and etched by effectively the sametechniques that are developed for the dielectric. The same equipment maybe used to deposit and etch both the perovskite electrode and thedielectric. These structures may also be used for multilayer capacitorsand other thin-film ferroelectric devices such as pyroelectricmaterials, non-volatile memories, thin-film piezoelectric and thin-filmelectro-optic oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asother features and advantages thereof, will be best understood byreference to the detailed description which follows, read in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a graph depicting the effect of La donor doping on theroom-temperature conductivity and the grain size of BaTiO₃;

FIGS. 2, 3, 4, 5, and 6 are cross-sectional views of a method forconstructing a capacitor with a lightly donor doped perovskite lowerelectrode on a semiconductor substrate;

FIG. 7 is a cross-sectional view of a high-dielectric-constant materialformed on a lightly donor doped perovskite; and

FIGS. 8, 9, and 10 are cross-sectional views of capacitors with lightlydonor doped perovskite lower electrodes formed on the surface of asemiconductor substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, there is shown a graph depicting the effect ofLa donor doping on the room-temperature conductivity and the grain sizeof bulk BaTiO₃ (BT). BT is generally considered to be a model perovskitematerial, and similar perovskites such as SrTiO₃ (ST) and (Ba,Sr)TiO₃should behave similarly. The bulk resistivity of the lightly donor dopedperovskite is generally between about 10 to 100 ohm-cm. Also, thisresistivity may be further decreased if the perovskite is exposed toreducing conditions. The perovskite can also achieve a high resistivity(about 10¹⁰ to 10¹⁴ ohm-cm) for large donor concentrations. The amountof donor doping to make the material conductive (or resistive) isnormally dependent on the process conditions (e.g. temperature,atmosphere, grain size, film thickness and composition).

As used herein, the term “lightly”, when used in reference to doping ofa perovskite, means a level of doping which produces a substantiallylower resistivity than that of an undoped version of the perovskite(e.g. a doping between about 0.01 and about 0.3 mole percent). Generallysuch lower resistivity is also substantially lower than the resistivityof a heavily doped version of the perovskite. Donor doping, as known inthe art, is generally the substitution of atoms on lattice sites which,due to the higher valence of the substituting atoms (as compared to thevalence of the atoms being replaced), results in free electrons.

With reference to FIGS. 2-6, there is shown a method of forming apreferred embodiment of this invention, a capacitor comprising ahigh-dielectric-constant material and a lightly donor doped perovskitelower electrode. FIG. 2 illustrates a silicon semiconductor substrate30. FIG. 3 illustrates an SiO₂ insulating layer 32 formed on the surfaceof the silicon substrate 30. FIG. 4 illustrates a lightly La donor dopedBST layer 34 deposited on the SiO₂ layer 32. This lightly La donor dopedBST layer 34 is conductive and will serve as the lower electrode for thehigh-dielectric-constant capacitor. FIG. 5 illustrates the capacitordielectric, a layer of undoped high-dielectric-constant BST 36,deposited on the lightly La donor doped BST layer 34. Although undopedBST may be used for the capacitor dielectric, acceptor doped or heavilydonor doped BST may also be used to provide a high-dielectric-constant.FIG. 6 illustrates the TiN upper electrode 38 deposited on the undopedBST layer 36. TiN is generally a good sticking layer and diffusionbarrier, in addition to being conductive. Alternatively, another layerof lightly La donor doped BST could be used instead of TiN for the upperelectrode 38.

In an alternate embodiment, FIG. 7 illustrates a layer of undopedhigh-dielectric-constant BST 36 deposited on a lightly La donor dopedBST layer 34. The lightly La donor doped BST layer 34 provides achemically and structurally stable electrical connection to the undopedhigh-dielectric-constant BST layer 36.

In another alternate embodiment, FIG. 8 illustrates ahigh-dielectric-constant capacitor utilizing a lightly donor dopedperovskite electrode. The TiN upper electrode 38 overlays the undopedBST layer 36, which in turn overlays the lightly La donor doped BSTlower electrode 34. However, the lightly La donor doped BST 34 is notformed directly on the first SiO₂ insulating layer 32, but is insteadshown formed on a TiN electrode buffer layer 42. The TiN electrodebuffer layer 42 is used as a sticking layer and diffusion barrier forsilicon, oxygen and impurities in the high-dielectric-constant BST layer36. Other materials such as RuO₂/Ru can also be used instead of TiN forthat purpose. For example, Ru metal could be deposited, and would forthe most part form RuO₂ during the deposition of the lightly La donordoped BST layer 34 or of the undoped high-dielectric-constant BST layer36. In this embodiment the TiN electrode buffer layer 42 is not used fordirect electrical connection since electrical contact is made directlyto the lightly La donor doped BST layer 34 from above, via a conductivetungsten plug 46. The tungsten plug 46 makes electrical contact to thealuminum top metallization 48 through the second SiO₂ insulating layer44. The two other tungsten plugs 46 make electrical contact from thealuminum top metallization layer 48 to the TiN upper electrode 38 and tothe doped silicon region 40.

In another alternate embodiment, FIG. 9 illustrates ahigh-dielectric-constant capacitor utilizing a lightly donor dopedperovskite electrode. As in FIG. 8, the lightly La donor doped BST lowerelectrode 34 is again formed on a TiN electrode buffer layer 42.However, in FIG. 9, the TiN electrode buffer layer 42 provideselectrical connection to the doped silicon region 40 below it. TiN alsoworks relatively well in this embodiment because it must undergosubstantial oxidation before it forms an insulating titanium oxide. Forexample, TiON and TiO are conductive, although TiO₂ is insulating.

The deposition of the lightly La donor doped BST lower electrode 34 ispreferably performed in a slightly reducing atmosphere when utilizingthe TiN lower electrode buffer layer 42 in order to minimize theoxidation of the TiN. The deposition of the undopedhigh-dielectric-constant BST layer 36 generally requires very oxidizingconditions and the lightly La donor doped BST lower electrode 34 willsignificantly slow the oxidation rate of the TiN electrode buffer layer42, thus inhibiting the formation of a substantially oxidized continuousresistive contact layer. Another benefit of this electrode system isthat the lightly donor doped BST lower electrode does little, if any,reduction the undoped BST layer 36.

In yet another alternate embodiment, FIG. 10 illustrates ahigh-dielectric-constant capacitor utilizing a lightly donor dopedperovskite electrode. As in FIG. 9, the TiN electrode buffer layer 42 isused for electrical contact. However, in FIG. 10, the TiN electrodebuffer layer 42 connects to the doped silicon region 40 via a tungstenplug 50.

Alternatively, the tungsten (or TiN) plug 50 in FIG. 10 could also beused to connect directly to the lightly La donor doped BST lowerelectrode 34, if the TiN electrode buffer layer 42 were not used.However, this would generally utilize the lightly La donor doped BSTlower electrode 34 as an oxygen diffusion barrier and hence may notprotect the tungsten plug 50 under all possible oxidizing conditions.

The sole Table, below, provides an overview of some embodiments and thedrawings.

TABLE Preferred or Drawing Generic Specific Other Alternate Element TermExamples Examples 30 Substrate Silicon Other single componentsemiconductors (e.g. germanium, diamond) Compound semiconductors (e.g.GaAs, InP, Si/Ge, SiC) Ceramic substrates 32 First level Silicon dioxideOther insulators insulator (e.g. silicon nitride) 34 Lower 0.1 to 0.20.01 to 0.29 mol % La electrode mol % La doped barium strontium dopedbarium titanate strontium Other lightly donor (e.g. F, titanate Cl, V,Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ta, W) dopedperovskite, ferroelectric, or high-dielectric-constant oxides (e.g.(Ba,Sr,Pb)(Ti,Zr)O₃, bismuth titanate, potassium tantalate, leadniobate, potassium niobate, lead zinc niobate, lead magnesium niobate)36 High- Undoped Other undoped perovskite, dielectric- bariumferroelectric, or high- constant strontium dielectric-constant oxidesmaterial titanate (e.g. (Ba,Sr,Pb)(Ti,Zr)O₃, (Pb,La)(Zr,Ti)O₃, bismuthtitanate, potassium tantalate, lead niobate, potassium niobate, leadzinc niobate, lead magnesium niobate) Acceptor (e.g. Na, Al, Mn, Ca, K,Cr, Mn, Co, Ni, Cu, Zn, Li, Mg) and/or heavily (generally greater than0.25 mol %) donor (e.g. F, Cl, V, Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd,Tb, Dy, Ho, Er, Ta, W) doped perovskite, ferroelectric, orhigh-dielectric-constant oxides (e.g. (Ba,Sr,Pb)(Ti,Zr)O₃, bismuthtitanate, potassium tantalate, lead niobate, potassium niobate, leadzinc niobate, lead magnesium niobate) 38 Upper Titanium Other conductivemetal electrode nitride compounds (e.g. nitrides: ruthenium nitride, tinnitride, zirconium nitride; oxides: ruthenium dioxide, tin oxide,titanium monoxide) Noble metals (e.g. platinum, palladium, rhodium,gold, iridium, silver) May be same materials as those listed for DrawingElement 34 above Other common semiconductor electrodes (e.g. silicides,aluminum) May contain more than one layer 40 Conductive Doped siliconSemiconductor devices semiconductor material 42 Electrode Titanium Otherconductive metal buffer layer nitride compounds (e.g. nitrides:ruthenium nitride, tin nitride, zirconium nitride; oxides: rutheniumdioxide, tin oxide, titanium monoxide, TiON silicides: titaniumsilicide) Combinations of above mentioned materials (e.g. TiN/TiO/TiON,TiN/TiSi, Ru/RuO/RuO₂) Other high temperature conductive diffusionbarriers This layer may or may not be used 44 Second level Silicondioxide Other insulators insulator (e.g. silicon nitride) 46 ConductiveTungsten Other reactive metals plug (e.g. tantalum, titanium,molybdenum) Reactive metal compounds (e.g. nitrides: titanium nitride,zirconium nitride; silicides: titanium silicide, tantalum silicide,tungsten silicide, molybdenum silicide, nickel silicide; carbides:tantalum carbide; borides: titanium boride) Conductive carbides andborides (e.g. boron carbide) Aluminum, copper Single componentsemiconductors (e.g. single crystalline and polycrystalline silicon,germanium) Compound semiconductors (e.g. GaAs, InP, Si/Ge, SiC) 48 TopAluminum Other common metallization semiconductor electrodes (e.g.silicides, TiN) Two or more layers of metal and dielectric 50 CapacitorTungsten Other reactive metals plug (e.g. tantalum, titanium,molybdenum) Reactive metal compounds (e.g. nitrides: titanium nitride,zirconium nitride; silicides: titanium silicide, tantalum silicide,tungsten silicide, molybdenum silicide, nickel silicide; carbides:tantalum carbide; borides: titanium boride) Conductive carbides andborides (e.g. boron carbide) Aluminum, copper Single componentsemiconductors (e.g. single crystalline and polycrystalline silicon,germanium) Compound semiconductors (e.g. GaAs, InP, Si/Ge, SiC)

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims. With reference to the structures described, electricalconnections to such structures can be ohmic, rectifying, capacitive,direct or indirect, via intervening circuits or otherwise.Implementation is contemplated in discrete components or fullyintegrated circuits in silicon, germanium, gallium arsenide, or otherelectronic materials families. In general the preferred or specificexamples are preferred over the other alternate examples.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method of forming a microelectronic capacitorstructure on an integrated circuit, said method comprising: (a) forminga semiconductor substrate; (b) forming an electrically conductive bufferlayer on said semiconductor substrate; (c) forming a conductive donordoped perovskite layer having between about 0.01 and about 0.3 molepercent doping on said buffer layer; and (d) forming ahigh-dielectric-constant material layer on said perovskite layer,whereby said donor doped perovskite layer provides a chemically andstructurally stable electrical connection to saidhigh-dielectric-constant material layer.
 2. The method according toclaim 1, wherein said perovskite is selected from the group consistingof: (Ba,Sr,Pb)(Ti,Zr)O₃, bismuth titanate, potassium tantalate, leadniobate, lead zinc niobate, potassium niobate, lead magnesium niobate,and combinations thereof.
 3. The method according to claim 1, whereinsaid donor is selected from the group consisting of: F, Cl, V, Nb, Mo,La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ta, W, and combinationsthereof.
 4. The method according to claim 1, wherein saidhigh-dielectric-constant material layer is selected from the groupconsisting of: (Ba,Sr,Pb)(Ti,Zr)O₃, (Pb,La)(Zr,Ti)O₃, bismuth titanate,potassium tantalate, lead niobate, lead zinc niobate, potassium niobate,lead magnesium niobate, and combinations thereof.
 5. The methodaccording to claim 1, wherein said electrically conductive buffer layeris selected from the group consisting of: platinum, palladium, rhodium,gold, iridium, silver, ruthenium, titanium nitride, tin nitride,ruthenium nitride, zirconium nitride, ruthenium monoxide, rutheniumdioxide, tin oxide, titanium monoxide, TiON, titanium silicide, andcombinations thereof.
 6. The method according to claim 1, said methodfurther comprising forming an electrically conductive layer on saidhigh-dielectric-constant material layer.
 7. The method according toclaim 6, wherein said electrically conductive layer is selected from thegroup consisting of platinum, palladium, rhodium, gold, iridium, silver,titanium nitride, tin nitride, ruthenium nitride, zirconium nitride,ruthenium dioxide, tin oxide, titanium monoxide, titanium silicide,aluminum, and combinations thereof.
 8. The method according to claim 6,wherein said electrically conductive layer is a second donor dopedsecond perovskite having between about 0.01 and about 0.3 mole percentdoping.
 9. The method according to claim 8, wherein said secondperovskite is selected from the group consisting of:(Ba,Sr,Pb)(Ti,Zr)O₃, bismuth titanate, potassium tantalate, leadniobate, lead zinc niobate, potassium niobate, lead magnesium niobate,and combinations thereof.
 10. The method according to claim 8, whereinsaid second donor is selected from the group consisting of: F, Cl, V,Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ta, W, andcombinations thereof.
 11. The method according to claim 1, wherein saiddonor doped perovskite layer is doped between 0.1 and 0.2 mole percent.12. The method according to claim 1, wherein said donor doped perovskitelayer is doped between 0.01 and 0.29 mole percent.
 13. The methodaccording to claim 1, wherein said high-dielectric constant materiallayer is undoped.
 14. The method according to claim 1, wherein saidhigh-dielectric constant material layer is doped with an acceptormaterial selected from the group consisting of: Na, Al, Mn, Ca, K, Cr,Mn, Co, Ni, Cu, Zn, Li, Mg, and combinations thereof.
 15. The methodaccording to claim 14, wherein said high-dielectric constant materiallayer is doped to greater than about 0.25 mole percent doping with adonor material selected from the group consisting of: F, Cl, V, Nb, Mo,La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ta, W, and combinationsthereof.
 16. The method according to claim 1, wherein saidhigh-dielectric constant material layer is doped to greater than about0.25 mole percent doping with a donor material selected from the groupconsisting of: F, Cl, V, Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Ta, W, and combinations thereof.